This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Understanding of electron spin relaxation is key to predicting feasibility, and optimizing experiments for oximetry, in vivo EPR, and distance measurements by relaxation enhancement and pulsed dipolar spectroscopy (DQC and DEER). There is no theory of the frequency dependence of electron spin relaxation that fits available experimental data. There is also a scarcity of experimental data other than at X-band with which to compare theory. Because of the wide-spread use of nitroxyl radicals, and the availability of various isotopically-substituted nitroxyls, they are selected for this study of relaxation processes. This project investigates the frequency dependence of relaxation in two motional regimes (a) rapid tumbling as is typical of in vivo spectroscopy and (b) immobilized samples at cryogenic temperatures as are used for pulsed dipolar spectroscopy. The Eaton lab has pulse capability from 250 MHz to 34 GHz. ACERT offers 9, 17, 35 and 95 GHz with high power pulses and CW at 170 and 240 GHz. The Eatons have a set of nitroxyl compounds with [^15]N, [^14]N, [^2]H, and [^1]H selective labeling. Spin-labeled peptides and samples for pulsed dipolar spectroscopy are available from other projects at ACERT and DU.